The invention relates to an apparatus for the injection of microwave energy into a receptacle that is at least partially transparent to microwave energy, by means of a microwave transmitter and an open microwave guide connected to the microwave transmitter and situated in the direct vicinity of the receptacle.
Such a receptacle either can be made entirely of a material transparent to microwave energy, i.e., it can be formed, for example, by a quartz tube, or it can have a microwave-transparent window of such a material.
An apparatus of the kind described above which has a window is disclosed, for example, in German Pat. No. 31 47 986. The "open microwave guide" consists in this case of a "ladder-like" metal structure having two parallel stringers and numerous "rungs" at equal distances apart and at right angles to the stringers and connecting the latter together electrically. The "rungs" are connected alternately each with one or two central conductors. With such structures it is possible to transmit the microwave power fed to one end over the length of the structure in an approximately uniform manner into the interior of the receptacle. In microwave parlance such structures are also referred to as "slow-wave structures," which can also be described as "delay structures."
"Delay structures" are basically microwave conductors through which a bond electromagnetic wave travels in a particular direction. The electric power is in this case transported, in the form of the electrical field surrounding the structure, at the so-called "group velocity" v.sub.g. Independently of this the phase of the wave travels with the phase velocity v.sub.Ph. The two velocities are also independent in their sign, so that it can easily occur that group velocity and phase velocity run in opposite directions. The expression, "slow," in connection with the waveguide structure, can refer either to the phase velocity or to the group velocity. In general, the group velocity can be between 0 and the speed of light in a vacuum. The value "0" applies to a structure that consists of resonant circuits which are not coupled to one another and through which no wave is running. The velocity of light, however, is achieved in a smooth waveguide. The phase velocity, on the other hand, can also assume values that are greater than the velocity of light.
The group velocity is lowered by loading a straight waveguide with periodically disposed obstacles--i.e., the so-called "rungs" in the case of the ladder structure described above. Depending on how these obstacles are arranged, velocities between 1% and 100% of the velocity of light can be established in practice. An extensive discussion of the properties of periodic structures at microwave frequencies is to be found in A. S. Harvey, "IRE Transactions on Microwave Theory and Techniques", Jan. 1960.
A whole series of slow-wave structures for the injection of microwave energy into receptacles and/or materials is described in the literature.
Of particular interest is the fact that the electric field connected with a particular power flow along the structure evidently must become greater as the group velocity becomes lower, i.e., for a constant flow of power the following equation applies: EQU E.sup.2 .about.1/v.sub.g.
This field can become so great for low group velocities and can extend so far into space away from the guide structure that it is suitable for the production of nonthermal low-pressure plasmas. Such an application is discussed, for example, by the group of authors, E. S. Hotston, J. M. Weaver, and D. J. H. Wort, in "U.K.A.E.A. Research Report," CLM-R78, 1968. This is a theoretical discussion having to do with the input of microwave energy into a plasma. The energy in this case is coupled in by a slow-wave structure located outside of the plasma. The plasma itself is confined by a magnetic field.
U.S. Pat. No. 3,663,858 discloses the production of a low-pressure plasma in a plasma receptacle with a slow-wave structure of helical configuration.
To make the energy input uniform over the length of the structure, it is proposed both in the first-named German Pat. No. 31 47 986 and in U.S. Pat. Nos. 3,472,200 and 3,814,983 to arrange the structure or structures at an acute angle to the adjacent material. When a receptacle is used, a uniform plasma is to be achieved over the enitre length of the receptacle. The effectiveness and limits of this method are described in German Federal Pat. No. 31 47 986 so that, to eliminate the undesired influences, it is proposed to use two structures crossing one another at an acute angle with contrary energy injection. For a length of the excited plasma of about 450 mm, this known solution resulted in a uniformity sufficient for many applications, which uniformity varies between .+-.5% and .+-.10% depending on the monomer used for the plasma process. At the same time, however, practical operation shows a number of limitations of the known solution:
Owing to its complexity, it is a relatively complicated and expensive apparatus. PA0 Since the substrate passes successively through two plasma zones of, as a rule, different strength, a change in the build-up of the coating is the result. PA0 The structure length cannot be increased indefinitely, because in this case the difficulty arises that, as the structure length increases, the necessary angle relative to the microwave window becomes increasingly acute. This leads not only to mechanical but also to electrical instability. PA0 On account of the division of the plasma length, which is needed for great substrate widths, into two or three structures arranged in tandem, the phenomenon of so-called "plasma bellies" occurs in the optically visible plasma, which are also reflected in an uneven distribution of the coating density. The reason for this phenomenon is not yet understood.